The analysis of fluid samples from hydrocarbon wells for the determination of phase behaviour and chemical composition is a critical step in the evaluation of the producibility and economic value of the hydrocarbon reserves. An important factor in determining the economic value of gas and liquid hydrocarbon reserves is their chemical composition, particularly the concentration of gaseous components, such as carbon dioxide, hydrogen sulfide and methane. Similarly, the monitoring of fluid composition during production operations can have an important bearing on reservoir management decisions, such as ceasing production from certain zones or applying chemical treatments to producing wells, e.g., biocides to kill the sulfate-reducing bacteria that generate hydrogen sulfide in the near-wellbore region.
Several disclosures have described the analysis of specific gases in borehole fluids in the downhole environment using near-infrared (λ=1–2.5 μm) spectral measurements. U.S. Pat. No. 5,859,430 describes the use of near-infrared spectroscopy to determine quantitatively the presence of methane, ethane and other simple hydrocarbons in the gas phase. The gases were detected using the absorption of near-infrared radiation by the overtone/combination vibrational modes of the molecules in the spectral region 1.64–1.75 μm. More recently, U.S. Pat. No. 6,465,775 describes a method of determining the concentration of carbon dioxide in the gas phase of a wellbore fluid sample using near-infrared spectroscopy in the spectral region 1.50–2.10 μm (particularly the spectral region 1.92–2.10 μm). The radiation was generated using either a narrow band LED or laser or a broad band thermal source (e.g. a tungsten halogen lamp) with appropriate filters. The large variation in the concentration of carbon dioxide that can be experienced in wellbore fluids was accommodated using optical cells of varying path lengths. The effects of temperature on the performance of the sources, detectors and filters was compensated for by the use of a reference sample having a well-defined absorbance at each sampling temperature.
WO 01/63094 describes several methods to determine the concentration of hydrogen sulfide in wellbore fluids. One method involved the generation of a headspace above a liquid sample with the purpose of determining the concentration of hydrogen sulfide in the gas phase and hence, by means of Henry's Law, the concentration of hydrogen sulfide dissolved in the original liquid sample.
WO 02/084334 describes the use of both near- and mid-infrared spectroscopy to determine the concentration of CO2 and hydrocarbons in gas samples in a borehole. The gas sample was obtained from a liquid by reducing the pressure to generate a headspace above the sample. The optical absorbance of the gas was measured either by transmission or internal reflection and a method of maintaining the transmissivity of the optical windows using wipers was described.
Smith and coworkers (16, 29) have shown the mid-infrared absorption spectra of the gases methane, carbon dioxide, nitrous oxide, nitrogen dioxide, carbon monoxide, nitric oxide, sulfur dioxide, ozone, ammonia and water vapour over the spectral region 2.5–12 μm. A number of LEDs have been fabricated with the aim of using them for the quantitative detection of various gases.